In the near- to medium-term future hydrogen production will continue to rely on fossil fuels, primarily, natural gas (NG). On the other hand, conventional hydrogen production processes are among major sources of anthropogenic CO2 emissions into the atmosphere.
In principle, hydrogen can be produced from hydrocarbon fuels via oxidative and non-oxidative conversion processes. Oxidative conversion involves the reaction of hydrocarbons with oxidants: water, oxygen, or combination of water and oxygen (steam reforming, partial oxidation and autothermal reforming processes, respectively). As a first step, these processes produce a mixture of hydrogen with carbon monoxide (synthesis-gas), which is followed by gas conditioning (water gas shift and preferential oxidation reactions) and CO2 removal stages. The total CO2 emissions from these processes (including stack gases) reaches up to 0.4 m3 per each m3 of hydrogen produced. Non-oxidative route includes thermal decomposition (TD) (or dissociation, pyrolysis, cracking) of hydrocarbons into hydrogen and carbon.
TD of natural gas has been practiced for decades as a means of production of carbon black with hydrogen being a supplementary fuel for the process (Thermal Black process). In this process hydrocarbon stream was pyrolyzed at high temperature (1400° C.) over the preheated contact (firebrick) into hydrogen and carbon black particles. The process was employed in a semi-continuous (cyclic) mode using two tandem reactors. U.S. Pat. No. 2,926,073 to P. Robinson et al. describes the improved apparatus for making carbon black and hydrogen from hydrocarbons by continuous thermal decomposition process. Kvaerner Company of Norway has developed a methane decomposition process which produces hydrogen and carbon black by using high temperature plasma (CB&H process disclosed in the Proc. 12th World Hydrogen Energy Conference, Buenos Aires, 697, 1998). The advantages of the plasmochemical process are high thermal efficiency (>90%) and purity of hydrogen (98 v. %), however, it is an electric energy intensive process. Steinberg et al. proposed a methane decomposition reactor consisting of a molten metal bath (Int. J. Hydrogen Energy, 24, 771, 1999). Methane bubbles through molten tin or copper bath at high temperatures (900° C. and higher). The advantages of this system are: an efficient heat transfer to a methane gas stream, and, ease of carbon separation from the liquid metal surface by density difference. A high temperature, regenerative gas heater for hydrogen and carbon production from NG has been developed by Spilrain et al. (Int. J. Hydrogen Energy, 24, 613, 1999). In this process, thermal decomposition of NG was conducted in the presence of a carrier gas (N2 or H2) which was pre-heated to 1627–1727° C. in the matrix of a regenerative gas heater.
There have been attempts to use catalysts to reduce the maximum temperature of the TD of methane. Transition metals were found to be very active in methane decomposition reaction; however, there was a catalyst deactivation problem due to carbon build up on the catalyst surface. In most cases, surface carbon deposits were combusted by air to regenerate the original catalytic activity. As a result, all carbon was converted into CO2, and hydrogen was the only useful reaction product. For example, Callahan describes a catalytic reactor (fuel conditioner) designed to catalytically convert methane and other hydrocarbons to hydrogen for fuel cell applications (Proc. 26th Power Sources Symp. Red Bank, N.J., 181, 1974). A stream of gaseous fuel entered one of two reactor beds, where hydrocarbon decomposition to hydrogen took place at 870–980° C. and carbon was deposited on the Ni-catalyst. Simultaneously, air entered the second reactor where the catalyst regeneration occurred by burning coke off the catalyst surface. The streams of fuel and air were reversed for another cycle of decomposition-regeneration. The reported process did not require water gas shift and gas separation stages, which was a significant advantage. However, due to cyclic nature of the process, hydrogen was contaminated with carbon oxides. Furthermore, no byproduct carbon was produced in this process. U.S. Pat. No. 3,284,161 to Pohlenz et al. describes a process for continuous production of hydrogen by catalytic decomposition of a gaseous hydrocarbon streams. Methane decomposition was carried out in a fluidized bed catalytic reactor in the range of temperatures from 815 to 1093° C. Supported Ni, Fe and Co catalysts (preferably Ni/Al2O3) were used in the process. The coked catalyst was continuously removed from the reactor to the regeneration section where carbon was burned off, and the regenerated catalyst was recycled to the rector. U.S. Pat. No. 2,476,729 to Helmers et al. describes the improved method for catalytic cracking of hydrocarbon oils. It was suggested that air is added to the feedstock to partially combust the feed such that the heat supplied is uniformly distributed throughout the catalyst bed. This, however, would contaminate and dilute hydrogen with carbon oxides and nitrogen.
Use of carbon catalysts offers the following advantages over metal catalysts: (i) no need for the regeneration of catalysts by burning carbon off the catalyst surface; (ii) no contamination of hydrogen by carbon oxides; and, (iii) carbon is produced as a valuable byproduct of the process. Earlier, Muradov has reported on the feasibility of using different carbon catalysts for methane decomposition reaction (Proc. 12th World Hydrogen Conf., Buenos Aires, Argentina, 1998). It has also been taught to thermally decompose hydrocarbon feedstock over carbon particles acting as a heat carrier. U.S. Pat. No. 2,805,177 to Krebs describes a process for producing hydrogen and product coke via contacting a heavy hydrocarbon oil admixed with a gaseous hydrocarbon with fluidized coke particles in a reaction zone at 927–1371° C. Gaseous products containing at least 70 v. % of hydrogen were separated from the coke, and a portion of coke particles was burnt to supply heat for the process; the remaining portion of coke was withdrawn as a product. U.S. Pat. No. 4,056,602 to Matovich deals with high temperature thermal reactions, including the decomposition of hydrocarbons, by utilizing fluid wall reactors. Thermal decomposition of methane was conducted at 1260–1871° C. utilizing carbon black particles as adsorbents of high flux radiation energy, and initiators of the pyrolytic dissociation of methane. It was reported that 100% conversion of methane could be achieved at 1815° C. at a wide range of flow rates (28.3–141.5 l/min). U.S. Pat. No. 5,650,132 to Murata et al. produces hydrogen from methane and other hydrocarbons by contacting them with fine particles of a carbonaceous material obtained by arc discharge between carbon electrodes and having an external surface area of at least 1 m2/g. Carbonaceous materials also included: soot obtained from the thermal decomposition of different organic compounds or the combustion of fuels; carbon nanotubes; activated charcoal; fullerenes C60 or C70; and, finely divided diamond. The optimal conditions for methane conversion included: methane dilution with an inert gas (preferable methane concentration: 0.8–5% by volume); A temperature range of 400–1,200° C.; and residence times of −50 sec. An increase in methane concentration in feedstock from 1.8 to 8 v. % resulted in a drastic drop in methane conversion from 64.6 to 9.7% (at 950° C.). It was also stated that during hydrocarbon pyrolysis (the experiments usually ran for 30 min) the carbon samples gradually lost their catalytic activity. It was suggested that oxidizing gases like H2O or CO2 be added to the pyrolyzing zone to improve the catalyst life. However, this would inevitably contaminate hydrogen with carbon oxides and require an additional purification step. Also, it was suggested that the spent catalyst be combusted, which would be, however, very wasteful, especially, considering the high cost of the carbon materials used in the process. U.S. Pat. Nos. 1,528,905; 2,367,474; 4,256,606; 4,615,993; 5,300,468 and 5,254,512 taught the different methods of regeneration of spent carbonaceous materials (CM), including activated carbons. However, these methods were mostly concerned with the reactivation of CM via removal (or displacement or decomposition) of the impurities (or adsorbable substances) from the surface of CM.
In summary of the foregoing, the major problem with the decomposition of methane (or other hydrocarbons) over carbon (or any other) catalysts relates to their gradual deactivation during the process. This could be attributed to two major factors: (i) loss of active surface area; and, (ii) inhibition of the catalytic process by the deposition of carbon species which are less catalytically active than the original carbon catalyst.
Thus, the need exists for a more effective, versatile and cost effective process for CO2-free production of hydrogen and carbon from different hydrocarbons using inexpensive and readily available catalytic materials.